Recently, silicon nitride draws attention because of its heat resistance, strength, and corrosion resistance. End products are generally obtained by molding silicon nitride powder into a compact of desired shape and sintering the compact. If the molded compact were increased in density, the sintered product would also have an increased density with less shrinkage, which is advantageous in strength and dimensional precision.
Silicon nitride powder is conventionally prepared by silica reduction, imide decomposition or direct nitriding of metallic silicon powder. Silicon nitride powders obtained by these conventional methods are molded into compacts, which cannot be increased in density beyond a certain limit, for example, in excess of 1.70 g/cm.sup.3.